This invention relates to fibrils. It more particlarly refers to carbon/graphite fibrils and to an improved process for producing such. Carbon fibrils as used herein means graphitic fibrils having high surface area, high Young""s modulus of elasticity and high tensile strength which are grown catalytically from available sources of carbon.
It has been known for some time that one could make fibrils by decomposing various carbon contributing molecules, such as light hydrocarbons, in contact with a suitable metal catalyst, such as for example iron alone or in combination with other metals. In the past, the fibrils which have been made have been somewhat thicker than desirable and/or have been burdened with an overcoat of thermally deposited generally amorphous carbon which tended to reduce the desirable physical properties thereof or have been made in poor yields. Prior workers have sought to ameliorate the disadvantages of the amorphous carbon overcoating by subjecting the finished fibril to a very high temperature graphitizing treatment whereby generally rendering the fibrils of substantially greater cross sectional consistency from both a composition and a crystallinity point of view.
It is obvious that the improved fibril properties engendered by this high temperature graphitization process are expensive, because high temperature treatments are expensive. Additionally, such graphitized fibrils may still be too thick for many purposes because, graphitizing does not significantly reduce the fibril diameter. Thus, it is desired to produce high yields of high quality fibrils, preferably thin fibrils, which, in a preferred aspect of this invention, do not need post production graphitization.
More particlarly refers to carbon/graphite fibrils and to an improved process for producing such. Fibrils are made according to this invention in a high temperature, catalytic process. The Fibril can be made of a variety of materials, e.g. carbon, silicon nitride, silicon carbide, etc. In one important embodiment, such fibrils have the atoms in their composition relatively ordered at their outer surfaces as they are made by this process. Thus, it can be said that this process preferably directly produces a product having a relatively crystalline outer region for substantial portions of its length and may have inner regions where its atoms are less ordered. It may, and often does, even have a hollow region axially positioned along substantial portions of its length.
Fibrils according to this invention are characterized by small diameters, e.g. about 35 to 70 nanometers and high L/D up to about 100 and even more. Where the preferred structure described above is produced, it is suitably produced directly in the fibril forming process without further processing being required.
Where the fibrils of this invention are to be made of carbon, such can be produced in quite high yields. In this embodiment, a suitable source of carbon may be a hydrocarbonaceous material illustrated by: methane, ethane, propane, butane, benzene, cyclohexane, butene, isobutene, ethylene, propylene, acetylene, toluene, xylene, cumene, ethyl benzene, naphthalene, phenanthrene, anthracene, formaldehyde, acetaldehyde, acetone, methanol, ethanol, carbon monoxide, other similar materials, and mixtures of two (2) or more thereof. Such feed is contacted with a suitable, catalyst at elevated, fibril forming temperatures for a time sufficient to cause graphitic carbon fibrils to grow.
It is within the scope of this invention to provide a non-hydrocarbonaceous gas along with the carbon contributing reactant. Such gas might for example be hydrogen or carbon monoxide. Inert diluents are also suitable.
The temperature of the process of this invention can vary widely depending upon the nature of the carbon source being used, however, for best results, it should be kept below the thermal decomposition temperature thereof. In the case of using a mixture of such carbon sources, the operating temperature should be maintained below the thermal decomposition temperature of the most temperature-sensitive carbon source in the system. Temperatures in the range of 500 to 1500xc2x0 C. may be found to be generally usable, depending on the carbon source used, preferably between about 600 and 900xc2x0 C.
Subatmospheric, atmospheric and/or super atmospheric pressures may be used as dictated by other processing considerations. It has been found that it is desirable to provide the carbon source in the vapor state, and thus, the pressure should not be so high as to cause the carbon source to be in the liquid state under fibril forming temperature conditions. Further, it is desirable although not essential to provide a suitable gaseous diluent, such as hydrogen or inert gases, for example, nitrogen.
It is preferred that the system as a whole be non-oxidizing wherefor preferably avoiding the presence of oxygen if practical. Small amounts of these materials can be tolerated. It should be understood that the existence of oxidizing conditions, at the elevated temperatures operative for this process, will cause oxidation of the carbon source and therefor reduce the amount of carbon from such source which is available for conversion into fibrils as desired.
It may be desirable to provide suitable heat to this reaction system where and when needed. Temperature of different parts of the reactor zone may be suitably controlled to different temperatures and this is easily accomplished by using electrical resistance heating. However in larger scale industrial practice, electric resistance heating may sometimes be economically replaced by direct heating, such as for example by burning some of the carbon contributing feed to raise the temperature of the remainder of the feed, or by feeding the catalyst or the carbon contributing feed, or the diluent into the system at a sufficiently elevated temperature such that direct heat exchange of the component with each other will cause the fibril forming reaction to proceed as desired.
The nature of the catalyst seems to have a significant effect upon the yield of fibrils produced according to this invention. It is known to use iron group metals such as iron, cobalt or nickel to catalyze the conversion of carbon contributing compounds to fibrils, and such metals are within the scope of this invention. In addition, many other multivalant transition metals, including lanthanides, appear to be operative. Particularly useful catalytic metals include inter alia: iron, molybdenum cobalt, nickel, platinum, palladium, vanadium, and chromium. Of specific interest in this process are certain combinations of transition metals. Particularly useful combinations include iron and molybdenum, iron and chromium, copper and nickel, iron and platinum, iron and tin, iron and nickel, iron and manganese, and iron and cerium.
The yield of fibrils produced according to the practice of this invention appears to be related to the physical state of the catalyst used to produce such. According to this invention, it is important that the multivalent transition metal fibril forming catalyst be present on a suitable substrate as relatively discrete catalytic sites, each about 35 to 700A preferably 60 to 300A in size during fibril formation. These relatively discrete catalytic sites are produced by suitably applying the transition metal (in an appropriate state) to a substrate, suitably an inorganic substrate material which can include carbon/graphite.
The size of the substrate particle is a matter of some importance dependent upon the engineering of the process itself. For example, if the fibril formation is to take place in a fluid bed type of reaction zone, the substrate particle size will suitably be less than about 400 microns. If the fluid bed is an evullient bed of catalyst particles, particle sizes of about 50 to 300 microns have been found to be preferable. If the fluid bed is an evullient bed of fibrils containing small amounts of catalyst particles, i.e. up to about ten percent, these should preferably have a size of about 1 to 100 microns. If the fluid bed is a transport bed, either up flow or down flow, the catalyst carrying particles will suitably be less than about 10 microns, preferably less than about one micron.
It has been found that depositing one or more suitable transition metals on small particle substrates produces a catalyst well suited to use in this invention. The substrate is a material which can conveniently withstand the rigors of fibril formation conditions, e.g. temperatures of about 500 to 1500xc2x0 C. Suitable substrates include carbon, graphite, inorganic oxides, etc. The particular substrate will be matched to the particular transition metal(s) catalyst such that the metal is bound strongly enough to retard migration and agglomeration but not so strongly as to prevent or retard the transition metal from catalyzing fibril formation. Illustrative, inorganic oxides include alumina, silica, magnesia, silicates, aluminates, spinels etc. Mixtures can be used.
Thus, very small particle iron, such as might be produced by decomposition of iron compounds, can be deposited on very small particle alumina, e.g. fumed alumina having particle sizes of no larger than about 100 mesh. These alumina particles may be made up of individual crystallites which are on the order of about 50 to 200 A, which agglomerate to form particles having substantial available surface area sufficient to receive deposits of appropriately sized transition metal catalyst.
The substrate particles are suitably less than about 300 microns. They may be less than 1 micron in transport bed use. It appears that the transition metal reacts with the substrate crystallites such as to bond the metal to the substrate and fix its position, so as to prevent or retard catalyst agglomeration, at least for so long as it takes to contact the supported transition metal with the suitable carbon source at appropriate reaction conditions. Upon contact, the carbon source seems to pyrolyze on the catalytic site and the desirable morphology fibril grows therefrom.
As noted, the state of the transition metal catalyst site during fibril formation is important to the practice of this invention. Sometimes, it appears that this desirable catalytic site state as well as the state of the substrate carrier therefore is changing during the whole process hereof. Thus, the catalytic sites may agglomerate or disperse to some extent during the period from introduction into the reaction zone until the fibrils made by the process are recovered. At the time the fibrils are recovered, particles of transition metal catalyst which are sometimes recovered with the fibrils are of about 35 to 700A, preferably 60to 300A in size. Thus, it is believed that the size of the active catalyst site during fibril formation is substantially comparable to the diameter of the fibril being formed.
It appears that as fibril formation takes place, active catalyst sites become catalytically expended and need to be replaced. Additionaly, it has been found that the fibril forming process is more efficient and capable of better control if the catalyst is added to the reaction zone intermittently or continuously over substantially the entire course of the reaction, or at least a substantial portion thereof. It is possible that the catalyst containing substrate of this invention may ablate with use. That is, when a fibril is formed on a particular catalytic site, that fibril and its associated site may break off from the substrate, with or without some of the substrate, thereby exposing further catalytic sites which were previously inside the substrate particle. Thus, periodic or continuous addition of fresh catalyst is desirable
Thus, according to this invention, the fibril forming process hereof is preferably substantially continuous in that a suitable source of carbon, with or without carrier gas, and catalyst containing particles are continuously or intermittently fed to a reaction zone maintained at a fibril forming temperature appropriate to the carbon source being used; while fibril product, usually admixed with the remnants of the catalyst and sometimes substrate as well, are continuously or intermittently recovered.
The transition metal may be deposited on the substrate by any commonly used technique for accomplishing such deposition. Vapor deposition, sputtering and impregnation may all be suitable. In particular, it has been found to be expeditious to form a water solution or dispersion of the desired metal or metals, mix the water phase with appropriately sized substrate, and then precipitate the metal(s) onto the substrate, e.g. by evaporating the water or any other conventional means.
It is also within the scope of this invention to deposit the desired transition metal(s) from an organic (as opposed to aqueous) medium. Suitably the transition metal can be dissolved or suspended in such medium, for example, as an organometallic compound, and then impregnated onto and into a suitable substrate. The organic carrier medium is removed, leaving behind the impregnated, deposited transition metal.
After the transition metal is combined with the substrate as aforesaid, it may be important to treat this combination so as to activate it for this particular catalytic purpose, e.g., by heating it to separate the metal from other ligands, if any, in the deposition compound. It may also be necessary to adjust the size of the prepared catalyst to make it suitable for use in this invention. Comminution or agglomeration, e.g. by binding, may be desirable to produce particles of the proper size, i.e. of less than about 400 microns.
The catalyst of this invention may be put on the substrate hereof in any form or chemical oxidation state. It may be the oxide or have some other ligand. It may be reduced prior to use, but this is not necessary since the fibril forming reaction is a reducing environment and thus the transition metal will be reduced during, or immediately prior to, fibril forming use.
Fibrils which are very thin and long, diameters of about 3.5 to 70 nanometers and L/D of up to 100 or more, are produced using these catalysts. These fibrils, as produced by this process, without the necessity of further treatment, and without the coproduction of a thermal carbon overcoat, comprise a carbon layer generally concentric about an axis which comprises multiple essentially continuous layers of ordered carbon atoms, which preferably and usually are crystalline and graphitic. This, as produced, outer layer of ordered carbon atoms often surrounds an inner layer of less ordered carbon atoms. Most preferred products of this invention are high yields of high quality, thin fibrils of appropriate long length having substantially uniform, concentric, substantially continuous, ordered, multiple layers of carbon about an axial (inner core) region, which has a different composition/crystallinity and is preferably hollow. Such fibrils preferably have up to about 100 times, and more greater length than diameter, have diameters of up to about 700 angstroms and are substantially cylindrical graphite about a substantially hollow core as made and without having been treated at higher temperatures than the original fibril manufacturing temperature.
According to one apsect of this invention, operating with catalyst particles as herein set forth, yields of fibrils of greater than about 30 times the weight of transition metal in the catalyst are achievable. In many cases, particularly with mixed transition metals, yields of between 100 and 200 times the weight of transition metal in the catalyst have been achieved. It has been found that in comparable processes, combinations of transition metal catalysts have sometimes increased yields by a factor of as much as 2 or even more.